Inflatable Airbag Systems to Improve
Ship’s Attained Subdivision Index
Devu P. Chodankar1 (AM)
1. Student of Masters Course in Naval Architecture at University of Liege, Belgium (www.emship.eu)
This document describes and proposes different configurations of inflatable air chambers placed in different
compartments of (marine floating structures like) ships to reduce permeability to sea water of these compartments, in
the event of collision damage, thus improving the Attained Subdivision Index of the ship calculated as per SOLAS
regulations. The document further describes some supporting systems to practically achieve this objective of
installing and using such airbag systems on ships to prevent sinking of ships due to collision damage of the hull.
KEY WORDS: EMERGENCY BUOYANCY, DAMAGE
STABILITY, ANTI-SINKING, SAFETY, UNSINKABLE,
INFLATABLE, SOLAS
INTRODUCTION
This document introduces a system consisting of inflatable air
bags placed in different compartments of (marine floating
structures like) ships to reduce “permeability” to “predetermined
volume” of each of these compartments, so as to improve
damage stability upon water ingress due to collision damage of
the hull. The application can be used as standalone
“permanently” filled up air bags in void tanks etc., or a centrally
controlled air bag system integrated with a pneumatic system,
which is activated manually only upon collision damage.
Further the integrated system can be designed to be
autonomously controlled by a master computer which computes
the damage stability of the vessel on a real-time basis;
intelligently decides and executes decisions regarding which
compartments will have what value of permeability so that the
ship does not sink and will thus inflate or deflate airbags or
ballast the compartments in the ship.
Prior Art
19th Century and earlier
The problems of damage stability of ships and the solutions
offered to address those problems are as old as Naval
Architecture itself. The use of air bags of different types are
known to the industry in various forms like air bags for
passenger safety in cars, in marine life rafts, hot air balloons,
inflated rubber fenders, etc.
The use of inflatable air bags to increase buoyancy of sunken
ships during salvaging operations and also for improving
damage stability are known in the United States from the days
of wooden ships.
To quote Mr. John Beninger from his 1886 patent: “….I am also
aware that vessels have been provided interiorly with flexible
bags permanently attached and provided with an air forcing
engine for filling them and thus preventing the vessel from
sinking for want of teuable space for water, and this feature I
broadly disclaim.”
John Beninger, 1886
It was discovered that US Patent No. 350,184 – System for
preventing vessels from sinking - describes deployment, both
inside and outside of a vessels hull, a system of canvas bags
communicating by means of pipes with a common reservoir
containing compressed gas.
Francisco L. De Villa, 1900
Further, U.S. Patent No. 644,480 - Device for preventing vessels
from sinking – describes air tight cuboidal rubber compartments
distributed throughout the vessel inflated by compressed air.
Thereafter, it appears, the focus shifted to welded steel hulls
which provided much better safety against damage and the
subject did not develop any further for a long time.
Hermann Hasse, 1976
Much later in Europe, Patent No. DE 2525202 A1 – Inflatable
buoyancy tanks for boat, has compressed air bottle for tank
connected via valve which operates upon capsizing – describes
flexible buoyancy tanks which take up minimum stowage space
when not in use, are connected to compressed air bottles via
valves, and also has pressure gauge and pressure relief valves
for pressure control.
Lothar Koehler, 1985
Later, Patent No. DE 3338375 A1 – Hull for yacht with devices
for maintaining the buoyancy in the event of sea damage –
describes foldable inflatable bags of flexible elastic material
mounted in hollow spaces in the hull, connected to a propellant
gas created by heating effect.
Heinz Boerner, 1987
Patent No. DE 3530618 A1 – Equipment and methods for
preventing the sinking of ships – describes the use of inflatable
airbags curled up in upper ceiling angle, inflated by compressed
air and pressure probes to measure pressure difference.
Steven P. Van Derryt, 2008
Recently, Patent No. 7412939 B2 – Emergency Buoyancy
Systems – describes a system comprising of one or more
flexible inflatable bags disposed on an upper inboard bulkhead
of one or more vessel compartments connected to an integral
gas delivery system.
From the above information it is very clear that the idea of
inflatable air bags is in existence since a long time and has
developed over a period of time to suit the advancements in
technology. In the next decades, we may see materials like
graphene sheets or fabrics made of graphene nanotubes being
used for the purpose of inflatable airbags. However, the author
would not like to jump that far in this document.
DESCRIPTION
The Unsinkable Ship
Introduction
There is a huge scope to create a complete system controlled by
a central master computer which autonomously calculates the
damage stability of the vessel using real time signal processing,
determines which compartments are to be ballasted or which air
chambers are to be inflated and operates the system
autonomously to execute the decisions. This will be the 21st
century’s “Unsinkable Ship”. Pneumatically controlled
inflatable air chambers will be an important component of such
a system.
One can imagine how such a ship system can benefit humanity
by preventing loss of human lives and property.
A Case Study to Demonstrate the Effect of
Change of Permeability on the Attained
Subdivision Index
The General Arrangement of a “Non Ballastable Deck Cargo
Barge” is shown in (Fig. 1). A probabilistic damage stability of
this barge was roughly assessed in Maxsurf software for the
following two conditions to provide a simple case study:
Permeability of all spaces at 95%
In this condition, it is assumed that the permeability of all the
compartments in the hull of the barge is 95%. In other words,
it’s a normal barge without the system installed. A detailed
probabilistic calculation is attached in Appendix A.
The results show that the attained subdivision index of the
vessel is 0.688. In this case, for the fully loaded departure
condition, there are multiple results showing “trim limit
exceeded” and “vessel has sunk”.
This means the ship is very much vulnerable if it sails in fully
loaded departure condition, at a permeability of 95%.
Permeability of all spaces at 35%
In this condition, it is assumed that the permeability of all the
compartments in the hull of the barge is 35%. This reduction in
permeability is achieved by installing the inflatable air bag
system in any one of its proposed configurations. The results of
a detailed probabilistic calculation for this condition are
attached in Appendix B.
Chodankar
The results show that the attained subdivision index of the
vessel has improved to 0.983, simply by reducing the
permeability of the compartments by using the inflatable air bag
systems. In this case, for all operating conditions, the GZ
(righting lever) curves are satisfactorily completed.
The results thus demonstrate that for the conditions assumed in
this case study, the damage stability of the vessel is improved
through deployment of the inflatable air bag system.
Of course, this is just an approximate study of a very simple
case. To develop an industry usable product, a more detailed
study has to be taken up to assess the possibility of deploying
the air bag systems in more complicated compartments of
different types of ships.
Components of the System
The primary components which constitute the system managing
the intact, damage and dynamic intact/damage stability of a
vessel on a real-time basis will be as follows:
(1)
(2)
(3)
(4)
Inflatable air chambers
Master computer
Pneumatic system
Instrumentation
Outline features of these primary system components are
broadly discussed in the following sections.
Inflatable Air Chambers
Industry is familiar with the air bag systems in cars which
activate upon collision of the vehicle. The purpose of those bags
is to reduce human injury in cases of high speed collision.
A slightly different concept is used for marine applications.
When deployed on ships, the purpose of these air filled bags is
to reduce permeability of compartments, or in other words, to
reduce the percentage of empty volume in any compartment
available for water ingress. Water ingress, which is a result of
breach of integrity of the outer hull of a ship, reduces the
buoyancy of any compartment, thereby having an adverse effect
on the overall stability of the vessel. Usually, the permeability
of empty void spaces is considered to be 95% having regard to
the structural members inside the compartment.
The purpose of hollow air filled bags is to occupy as much
space as possible within any compartment, and thus to prevent
entry of water into the compartment. In technical terms, this
means the permeability of the compartment is reduced by the
amount of volume occupied by the air bag. Reduction of
permeability means that in an event of damage, lesser water
would enter the compartment and would therefore result in
lesser reduction in buoyancy of the vessel. The effect of this on
the stability of the vessel will have to be considered on a case to
case basis. In most cases, this should have a positive effect on
the stability of the vessel, thus preventing the vessel from
sinking.
Inflatable Airbag Systems to Improve Ship’s Attained Subdivision Index
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Deployment strategy
Earliest applications of such air bags envisaged deployment
both inside and outside the hull. Although deploying the
inflatable air bags outside the hull provides extra buoyancy, they
also interfere with other factors like resistance of the ship.
Conversely, when deployed inside a ship compartment there is
greater control on the relative center of gravity of the air bags
w.r.t. the ship. Also, when deployed inside a compartment the
air bags and their instrumentation are least affected by the
environmental factors. There is also no direct effect on the
resistance of the ship when deployed inside a compartment. So
deployment within a compartment is preferred for this study.
Broadly two types of air bags are proposed:
(1) Stand-alone inflatable air bag systems independently
deployed for each void tank, and
(2) Pneumatically controlled, self-activated air bag system
responding in real-time to collision damage.
Stand-alone inflatable air bag systems independently deployed
for each void tank
This system, as shown in (Fig. 2) will consist of one
independent air bag deployed in each void tank. Locations,
quantity and volume of air bags will have to be carefully
considered with due regard to damage stability calculation for
the vessel. Each air bag will be attached by strings of suitable
material, to the inboard side of the ship’s tank. Additional
supports may also be provided at deck bottom and side shell.
The supports on the inboard bulkhead and deck will be regarded
as the primary support members as the damage is less likely to
happen from these sides. The airbag should be able to retain
position even in case of partial or major damage of side shell
and / or bottom. The air bags should have tension members of
suitable materials inside, so as to retain shape when inflated.
The material of the air bag should be water tight, wear resistant
and resistant to piercing. They may even be internally
subdivisioned.
The air bag has to be attached inside the tank during the first
installation. Then it has to be inflated and kept inflated during
its lifetime. In case the need arises for inspection or maintenance
of the tank or of the air bag system, the bag should have a valve
to deflate it. There should also be a relief valve to release any
sudden pressure created during collision.
This system will be installed stand alone in each designated tank
based on prior calculations. All control valves will be available
on the most accessible deck above the tank. A portable
pneumatic pump should be available on the ship to cater the
need of the system.
Due to the limitation that the air bags of the system have to be
kept filled all the time, this system can be used only for void
tanks / unused spaces. Since any inflated air bags will not allow
proper filling of the ballast tanks, this system cannot be used for
ballast tanks or other spaces which require a substantial volume
of the spaces to be used for another purpose.
Chodankar
As shown in (Fig. 3), in case of collision damage, the relief
valve releases the excess pressure created by collision and
allows only the required reduction in volume of the air bag, such
that the air bag takes shape of the colliding object. If the object
is removed soon after collision, the air bag may be re inflated to
regain its original shape. This of course is a temporary solution
till the vessel can be taken to salvage / repair point and will
allow enough time for recue of passengers and crew.
Pneumatically controlled, self-activated air bag system
responding in real time to collision damage
As shown in (Fig. 4) this system will be similar to the stand
alone inflatable system, except that it will overcome the
limitation of using the system in other spaces like ballast tanks
and even engine room.
The internal construction of air bags will be similar to the stand
alone air bags. The method of installation of air bags within the
tanks will also be similar. Only difference will be that, wherever
required the air bag can be installed as an un-inflated (deflated)
air bag which, upon collision damage of the respective
compartment will respond by getting inflated. For this purpose
the system will be connected to a pneumatic system consisting
of sensors and a controlling computer. In case of collision
damage the sensors will identify collision damage and will
sound an alarm and inform the computer. The computer will
calculate the stability of the ship under damage condition and
will develop a strategy to correct the stability by inflating the
required air bags. Alternatively, the computer (or manual
command) can simply instruct the pneumatic system to restore
buoyancy of the damaged compartment only.
In engine room (which is a big unprotected compartment)
multiple air bags may be deployed to reduce permeability of
space below waterline. These air bags will be usually hidden in
their containers and will spring into action only upon collision
damage of the engine room.
Multiple airbags to tackle pressure gradient
Larger vessels will have a huge difference in pressure at deeper
levels as compared to that at the surface, as seen in (Fig. 5). Due
to this pressure difference, if a single air bag is used for the
entire depth of the vessel, the internal pressure of the air bag
may be too high at surface and insufficient to counter sea
pressure at greater depth. This may result in rupture of the air
bag at lower depths, due to high pressure and may also result in
shape deformation at deeper levels. This problem can be tackled
by providing multiple air bags at different levels, each supplied
separately from the main air line. The inlet point at each air
supply will have a pressure sensor with automatic cut off at
predetermined internal pressure. Thus air bags of different
strengths may be provided at each depth so as to reduce the
overall cost of the system.
Usually, the internal pressure of the air bags has to be sufficient
to counter the sea pressure at the respective depth of the vessel,
highest pressure being at the deepest level.
Inflatable Airbag Systems to Improve Ship’s Attained Subdivision Index
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Fig. 1 Case study – general arrangement of a non ballastable
deck cargo barge
Chodankar
Inflatable Airbag Systems to Improve Ship’s Attained Subdivision Index
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Fig. 2 Inflatable airbag system (before collision damage)
Fig. 3 Inflatable airbag system (after collision damage)
Chodankar
Inflatable Airbag Systems to Improve Ship’s Attained Subdivision Index
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Fig. 4 Inflatable airbag system (in deflated condition)
Fig. 5 Use of multiple airbags in deep tanks to counter pressure
gradient
Chodankar
Inflatable Airbag Systems to Improve Ship’s Attained Subdivision Index
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Fig. 6 Possible use of air bag protection for cargo oil to avoid
spillage in the event of collision damage to hull
Fig. 7 Possible use of air bags to prevent bulk cargo from
spilling, thus maintaining the permeability value
Chodankar
Inflatable Airbag Systems to Improve Ship’s Attained Subdivision Index
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Maintaining permeability value of cargo holds
Generally, cargo occupies a large volume on any cargo ship. In
the event of damage to the cargo hold, there is spillage of the
cargo and resultant water ingress. Spillage of oil creates an
environmental hazard. As far as the damage stability calculation
is concerned, the spillage of cargo changes the permeability of
the compartment, thereby making an accurate assessment of
stability difficult.
As shown in (Fig. 6-7), the cargo spaces of oil tankers / bulk
carriers may be lined with inflatable air bags to prevent spillage
of cargo in the event of collision. Generally in case of low
energy collisions, the double hull would protect the cargo from
spilling in double hull ships. But single hull vessels like bulk
carriers remain vulnerable to spillage of cargo. In case of higher
energy collisions, the air bags may provide a cushioning effect
by absorbing the energy of collision and releasing part of this
energy by way of pressure relief valves. Thus air bags can
prevent spillage of cargo, thereby maintaining the permeability
value of the cargo compartment unchanged.
Master Computer
Role of master computer
A centrally located computer will be in control of the entire
system, with suitable back up computer if required. This
computer is expected to intelligently assess the damage situation
and take remedial action to correct the stability of the vessel.
The computer will be connected to a system of sensors, which
provide all the required data like draft readings, tank soundings,
trim and heel values, environmental values, ship motion values,
etc. which will help the computer to assess the intact, damage
and dynamic stability of the vessel on a real time basis. Thus the
computer will be able to decide at any given point of time, if
any corrective measures like ballasting or deballasting are
required to improve the stability of the vessel.
The computer will also be connected to every system in a
controlling position to implement the remedial actions. Thus,
the computer will be able to automatically ballast or deballast
the tanks. It will be able to inflate or deflate the air bags to
change the permeability value of each tank as the situation
demands.
Pneumatic System & Instrumentation
Pneumatic system
This system will be used to inflate or deflate the air bags and
will consist of the necessary instrumentation like pressure
gauges, valves, relief valves, sensors etc. The capacity and
location of the system has to be decided on a case to case basis.
USES OF THE PROPOSED SYSTEM
The system may find universal application for different ship
types and designs. Once it is practically proven that the system
actually improves the stability and safety of the vessel in
damaged condition, the statutory authorities may provide a
relaxation in freeboard for the vessel to the extent of the weight
Chodankar
added by the system so that the vessels installing the systems do
not have to pay a penalty due to added weight.
It may also reduce the initial cost of a vessel if certain
dimensions can be reduced by virtue of the system. Ships may
become more efficient if freeboard relaxations are possible.
In defense sector, we may be able to develop the system to adapt
to actual combat situations so as to ensure that the vessels can
withstand actual damage during combat.
In passenger cruise ship sector, we might be able to ensure that
incidents like “Titanic” or “Costa Concordia” do not claim
human lives.
CONCLUSIONS
An “Unsinkable Ship” is a concept whose time has come. With
the use of proper materials and systems, and advanced
computers, we should be able to create ships which can
withstand major damage to the external hull. Control of
permeability of compartments is a useful tool to manage the
damage stability of vessels on a real-time basis. Careful control
of permeability of compartments within a ship can improve the
“Attained Sub-Division Index” of the ships.
ACKNOWLEDGEMENTS
Author wishes to express gratitude to SNAME T&R coordinator
Mr. Alexander C. Landsburg for providing valuable inputs to
improve technical and presentation aspects of the submission.
Author is also thankful to Mr. V.P. Kumar, Dr. Carmo Quadros
and Dr. Rigo Phillippe for providing guidance and inspiration.
REFERENCES
Beninger, John. System for preventing vessels from sinking, US
Patent No. 350,184 (1886)
Boerner, Heinz. Equipment and methods for preventing the
sinking of ships, Patent No. DE 3530618 A1 (1987)
De Villa, Francisco L. Device for preventing vessels from
sinking, U.S. Patent No. 644,480 (1900)
Hasse, Hermann. Inflatable buoyancy tanks for boat, has
compressed air bottle for tank connected via valve
which operates upon capsizing, Patent No. DE
2525202 A1 (1976)
Koehler, Lothar. Hull for yacht with devices for maintaining the
buoyancy in the event of sea damage, Patent No. DE
3338375 A1 (1985)
Van Derryt, Steven P. Emergency Buoyancy Systems –
describes a system comprising of one or more flexible
inflatable bags disposed on an upper inboard bulkhead
of one or more vessel compartments connected to an
integral gas delivery system. (2008)
APPENDIXES
Appendix A Case study – Probabilistic damage stability results
(permeability 95%)
Appendix A Case study – Probabilistic damage stability results
(permeability 35%)
Inflatable Airbag Systems to Improve Ship’s Attained Subdivision Index
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APPENDIX A
Chodankar
Inflatable Airbag Systems to Improve Ship’s Attained Subdivision Index
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Chodankar
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APPENDIX B
Chodankar
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Chodankar
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